1. Field of the Invention
This invention relates to improvements in a display system for optically projecting a display light of information to be observed by a person, and more particularly to the display system such as a head-up display system which is configured, for example, such that a driver of an automotive vehicle can observe an image of driving information projected as the display light on a windshield glass while looking a frontal view through the windshield glass in such a manner that the driving information is superimposed on the frontal view.
2. Description of the Prior Art
A variety of head-up display systems of an automotive vehicle have been hitherto proposed and put into practical use. A typical head-up display system is constituted by directly applying a film functioning as a half mirror on a windshield glass, or by interposing the half mirror between two glass plates. However, with this head-up display system, reflection of light of an image is unavoidably made on the front-side or rear-side surface of the glass plate thereby forming a double image which is difficult to be observed by a driver. In view of this, another types of the head-up display system have been proposed, in which light of the image is reflected from the surface of the glass plate without using the half mirror in order to avoid formation of the double image. One of these is configured by applying a so-called .lambda./2 film (for changing a direction of polarization of light) on a transparent plate such as a glass plate, as disclosed in Japanese Patent Provisional Publication No. 2-141720. Another one is configured by bonding a transparent film having birefringence on a transparent plate, as disclosed in Japanese Patent Provisional Publication No. 2-294615. A further one is configured by bonding a polarization-direction changing layer formed of a crystalline high polymer on a transparent plate, as disclosed in Japanese Patent Provisional Publication No. 6-40271. Additionally, some of the inventors of this application have proposed a display system including a combination of a polarization-direction changing film, a light-transmittable reflection film and an antireflection film, as disclosed in a U.S. patent application Ser. No. 08/621379 entitled "Glass Arrangement".
However, the display systems as disclosed in the above publications and application require to cause the light of image to be incident on the transparent plate at Brewster's angle of the transparent plate. Accordingly, the angle of incidence of light is fixed at 56.6.degree. in case that the transparent plate is a glass plate. As a result, location of a displaying device for generating the light of image is limited within a certain region. This is considerably problematic particularly in case that the display system forms part of an automotive vehicle which does not have a sufficient space within the above-mentioned certain region. In view of this, an additional optical device such as a mirror may be required as disclosed in Japanese Patent Provisional Publication No. 2-141720, complicating the display system in construction and in assembly process.